Trusting the client with all things is a bad idea. Running stuff like presentation/layout/gui on the client makes sense. For some things, it makes sense to run the same code first on the client, but the again on the server (so not really trusting the client). Example: form validation. You want to run it on the client, to give early feedback. You need to run it on the server, because you can't trust client-supplied data. Makes your life a lot simpler, if you only need to implement the validation code once, even though you use it both on the client and on the server.